A binocular rangefinder is known as device for determining the distance to an object and has been used to determine the distance between running cars and as a member of an autofocusing system for cameras or a three-dimensional shape measuring system.
Such a binocular rangefinder includes binocular optical systems, on each of which a light beam reflected from an object is incident, and its associated imagers that produce images based on the light beams that have been incident on the optical systems. By using such a binocular rangefinder, the imagers can produce images based on the light beams that have been incident on the binocular optical systems, and the distance to the object can be determined based on the parallax between those two images.
In a conventional binocular rangefinder, two lenses are arranged side by side either horizontally or vertically so that each of those lenses produces an image on its associated imager (see Patent Document No. 1, for example).
FIG. 10 illustrates how a binocular rangefinder carries out triangulation. As shown in FIG. 10, the light reflected from the object 9 is incident on first and second lenses (or optical systems) 3a and 3b, thereby producing the object's images on first and second image capturing planes 4a and 4b, respectively.
Suppose a measuring point is set at a point P on the object 9 and is located on the optical axis of the first lens 3a. In that case, the image of the point P will be produced at the intersection between the optical axis of the first lens 3a and the first image capturing plane 4a. On the second image capturing plane 4b, on the other hand, the image of the point P will be produced at a point that deviates from the optical axis of the second lens 3b in the base line direction by Δ, which is called the “magnitude of parallax”. In this case, the distance z from the first and second lenses 3a and 3b to the point P (as measured parallel to their optical axes), the base line length B that is the distance between the optical axes of the first and second lenses 3a and 3b, the focal length f of the first and second lenses 3a and 3b, and the magnitude of parallax Δ will satisfy the following approximation equation:z≈B·f/Δ  (1)
Since the base line length B and the focal length f are already known, the distance to the object 9 can be determined by extracting the magnitude of parallax Δ from the images. In this case, the magnitude of parallax Δ can be extracted by performing pattern matching between the image produced on the image capturing plane 4a through the first lens 3a and the one produced on the image capturing plane 4b through the second lens 3b. 
As the temperature of the environment surrounding the rangefinder varies, however, the respective members that form the rangefinder will expand or shrink to a certain degree. The more complex the structure of the rangefinder, the more significant the error to be caused by complicated shifts of the optical axes due to the temperature variation. In order to overcome such a problem, according to a proposed method, the temperature is measured with a temperature sensor arranged between the lenses, and the interval between the optical axes is calculated and corrected based on a variation in the temperature and the linear expansivity of the material of the respective members (see Patent Document No. 2, for example).                Patent Document No. 1: Japanese Patent Application Laid-Open Publication No. 4-43911        Patent Document No. 2: Japanese Patent Application Laid-Open Publication No. 10-232128        